In accordance with the phase separation method of particle production, for example, for production of latex particles, the latex material is disposed in a solvent, which in turn is suspended in droplets in a fluid bath. The solvent and bath materials are chosen such that the solvent is slightly soluble in the fluid bath material, but the latex is essentially insoluble in the bath. Thus, as solvent diffuses into the fluid bath material, the latex is continuously redistributed and concentrated. When all of the solvent has diffused out of the latex droplets, a solid latex particle remains, the size of which depends on the amount of latex material in the original droplet.
The prior art teaches numerous methods whereby the droplets of solvent used in the phase separation process are sized and processed such that particles of corresponding size and precision will be produced. For particles in the range of two to five micrometers in diameter and above (for example, to fifty to ninety micrometers or more), most prior art methods fail to assure a precision within 5% of the desired diameter.
It is an object of the present invention to provide in principle methods for producing microparticles in the range from approximately two to one hundred micrometers in diameter, at a degree of precision substantially better than the 5% tolerances demonstrated by most of the prior art.
The prior art does, however, teach at least one method which purports to produce monodispersed particles in the desired size ranges at a precision of at least 2% by volume. This method is described by M. J. Fulwyler et al. in an article entitled "PRODUCTION OF UNIFORM MICROSPHERES," Review of Scientific Instruments 44, 1973. Similar techniques are set forth in U.S. Pat. No. 4,162,282 to Fulwyler et al. entitled "METHOD FOR PRODUCING UNIFORM PARTICLES," issued July 24, 1979, from an application filed Apr. 22, 1976. These techniques are based on synchronized droplet formation principles first investigated by Lord Rayleigh in the nineteenth century to disintegrate a jet of solvent material contained within a sheath fluid, the solvent being soluble in the sheath fluid. In accordance with the Fulwyler et al. techniques, a core liquid is injected into a moving sheath liquid. When combined, sheath and core are together formed into biphasic droplets as the fluids are jetted from a vibrating nozzle. The droplets are collected, and by stirring are held suspended in a catch liquid until the core and the sheath liquids from each droplet have diffused into the catch liquid, leaving particles formed of the materials which were dispersed within the core.
It has been found that, in accordance with the Fulwyler et al. methods, the mechanism by which the fast-moving droplets are collected and stirred is an important, and indeed critical mechanism for the formation of uniform particles. The fast flowing sheath of fluid forms biphasic droplets which have considerable kinetic energy and momentum; depending on droplet size and velocity, there exists a varying degree of risk, nearly always substantial, that the droplets will experience shear forces of such intensities that they are broken apart during the collection process.
Utilization of the teachings of Fulwyler et al. to generate high precision microparticles on a time sustained basis therefore necessitates very substantial design compromises. Most basically, avoidance of particle breakup mitigates in favor of relatively large sheath-to-core diameter ratios, and the employment of relatively large, expensive, and ultimately wasteful amounts of sheath and catch fluids in order to generate a relatively small amount of particles. Moreover, regulation of the sheath-to-core ratio by reducing the diameter of the core injection nozzle tends to promote clogging unless the overall particle production rate is substantially decreased. Additionally, Fulwyler also must resort to application of a similar charge to each droplet to prevent coalescing during the formation process.
Primary objects of the present invention therefore include the utilization of the core/sheath approach, as taught by Fulwyler, but at faster rates, employing substantially reduced sheath and catch liquid volumes, eliminating the need for droplet charging, and employing respective nozzle sizes which obviate the danger of frequent clogging.